What is inflation?
Key takeaways
04 · ideas- Inflation = the same money buys less over time
- At 3% inflation, purchasing power halves every 24 years
- Measured by the Consumer Price Index (CPI) tracking ~80,000 prices
- Savings accounts earning less than inflation are losing real value
Basic level
The essentials · 1 min
Inflation means your money buys less over time. Prices gradually rise, so the same $100 gets you less groceries, less gas, and less of almost everything.
A concrete example:- 1990: A movie ticket cost $4.22
- 2024: That same ticket costs $13.00
- That's 208% more — driven by inflation over 34 years
- Your $100 today = $97 in purchasing power next year
- In 10 years = $74
- In 20 years = $55
- In 30 years = $41
Why inflation happens: Too much money chasing too few goods. When demand exceeds supply — or when the money supply grows faster than economic output — prices rise.
Winners and losers from inflation:- Losers: People holding cash, bonds with fixed payments, anyone on a fixed income
- Winners: Homeowners (home values rise), stock investors, borrowers with fixed-rate debt (paying back cheaper dollars)
Intermediate level
How it applies · 2 min
What the CPI measures
The Consumer Price Index (CPI) tracks prices monthly across 8 major categories:
| Category | Weight in CPI | Recent inflation contribution |
|---|---|---|
| Housing (shelter) | 36% | Often the largest driver |
| Food | 14% | Volatile, affected by weather/supply chains |
| Transportation | 15% | Heavily influenced by oil prices |
| Medical care | 7% | Consistently outpaces overall CPI |
| Education | 6% | Has risen faster than CPI for decades |
| Energy | 7% | Most volatile — swings ±30% in a year |
| Apparel | 3% | Has barely risen — globalization effect |
| Other | 12% | Mixed |
How inflation destroys fixed income
The impact of 3% annual inflation on $100,000 in cash:
| Year | Nominal value | Real purchasing power | Purchasing power lost |
|---|---|---|---|
| 0 | $100,000 | $100,000 | — |
| 5 | $100,000 | $86,261 | $13,739 |
| 10 | $100,000 | $74,409 | $25,591 |
| 20 | $100,000 | $55,368 | $44,632 |
| 30 | $100,000 | $41,199 | $58,801 |
The implication: Keeping money in a 0.5% savings account when inflation is 3% isn't "safe" — you're losing 2.5% of purchasing power per year. Over 20 years, you lose 40% of real value.
Inflation-beating investments (historical)
| Asset | Average annual return | Inflation hedge quality |
|---|---|---|
| US stocks (S&P 500) | ~10% nominal | Excellent (beats inflation by ~7%) |
| Real estate | ~8% | Good (rents and values rise with inflation) |
| TIPS (inflation bonds) | CPI + 0.5–2% | Perfect hedge (by design) |
| Gold | ~4–5% | Moderate (inconsistent) |
| Regular bonds | 4–5% | Poor (fixed payments lose value) |
| Cash/savings | 1–3% | Very poor (often below inflation) |
Advanced level
The math, step by step · 1 min
Measuring inflation: the index formula
The CPI is a Laspeyres price index:
Where p_t = current prices, p_0 = base period prices, q_0 = base period quantities.
Inflation rate:The Fisher equation and real returns
Nominal interest rate i, real interest rate r, inflation \pi:
Practical implication: A 4% bond when inflation is 3% earns only ~1% real return. A 2% savings account during 6% inflation earns -4% real return — your money is shrinking in purchasing power.
The quantity theory of money
Where M = money supply, V = velocity (how often money changes hands), P = price level, Q = real output. If the money supply grows faster than output (with constant velocity), prices must rise proportionally.
This is why central banks control money supply: printing money without corresponding economic growth causes inflation. Zimbabwe (2008: 89 sextillion %) and Venezuela (2018: 1,000,000%) are extreme examples.